systemdialog: also consider buttonbox for dimensions

otherwise the dialog may be shrunk beyond the space that the buttonbox
desires resulting in cut off buttons (only when the mainItem is small
enough, naturally)


(cherry picked from commit 1faeaf2e72)

lookandfeel/contents/systemdialog/SystemDialog.qml

@@ -1,5 +1,6 @@
 /*
  *  SPDX-FileCopyrightText: 2021 Devin Lin <espidev@gmail.com>
+ *  SPDX-FileCopyrightText: 2022 Harald Sitter <sitter@kde.org>
  *
  *  SPDX-License-Identifier: LGPL-2.0-or-later
  */
@@ -27,8 +28,8 @@ Item {
     property Window window
     implicitHeight: column.implicitHeight
     implicitWidth: column.implicitWidth
-    readonly property real minimumHeight: column.Layout.minimumHeight + mainItem.implicitHeight
-    readonly property real minimumWidth: column.Layout.minimumWidth + mainItem.implicitWidth
+    readonly property real minimumHeight: column.Layout.minimumHeight + mainItem.implicitHeight + footerButtonBox.implicitHeight
+    readonly property real minimumWidth: column.Layout.minimumWidth + mainItem.implicitWidth + footerButtonBox.implicitWidth
     readonly property int flags: Qt.Dialog
     property alias standardButtons: footerButtonBox.standardButtons
     readonly property int spacing: Kirigami.Units.largeSpacing // standard KDE dialog margins